Another great article from CAPA
http://centreforaviation.com/analysis/spirit-airlines-expansion--unit-revenue-declines-concern-wall-street-but-are-central-to-the-model-247097
I especially like this part
"This is in general unfortunate for the longer term outlook for the US industry; so long as airlines are forced to respond to often uneducated and unduly short term analysis, there are enormous pressures on management to conform to a simple "profit-now" profile. Where the typical low cost model, in markets where they have cost advantages, is to expand rather than driving yield up, that model risks corruption."